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Neurobiology of Hearing Salamanca, 22nd May 2019
3. Cochlear amplification: outer hair cells
Jonathan Ashmore Neuroscience, Physiology and Pharmacology University College London
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Mechano-electrical transduction in hair cells
• depends on a flow of K+ into the cells • adapts on a time scale of milliseconds • depends on mechanical coupling no 2nd messengers direct • depends on a fast channel opening (<1µs) • depends on an unknown channel?
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The mammalian cochlea is a mechanical spectrum analyser
A
Stape Basilar membrane s
Fluid Fluid in motion at Round rest window
B
IHC 3 rows OHCs
BM stiffness high low
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y/x relative x amplitude
y
spring & mass frequency
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y/x relative x amplitude
y
+ viscous damping frequency
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We can use this idea to make an in-silico model of the cochlea
See:
http://147.162.36.50/cochlea/cochleapages/theory/index.htm
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Problem: cochlear bandwidths are narrow..
Solution: a ‘cochlear amplifier’ to counteract viscosity effects (Gold 1948, Davis 1983)
responsible for sound amplification (100 x) responsible for frequency selectivity ‘linked’ to otoacoustic emissions
Use the outer hair cells to compensate for fluid viscosity
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the amplifier
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Kv7.4 / KCNQ4 – one of the main K channels in OHCs
Mutations in the gene define a deafness locus DFNA2
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Inside +ve Inside -ve
Outer hair cells have an additional property: they change length
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Two ways to make a cylindrical cell change length
solute influx PI PO Area constant
water efflux
Volume constant Vo Vm (The cochlear motor)
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Outer hair cells in situ distort the cochlear partition
Extracellular current
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The ‘motor’ occupies most of the OHC lateral membrane
100x magnification: 8nm diameter particles
B Kachar - NIDCD 13
The motor is local and has an electrical ‘fingerprint’
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The outer hair cell is an ultrafast motor / actuator
an area motor high copy number (>107 /cell) fast cycle time if driven ( > 50 kHz) operation associated with a gating charge
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The molecular motor, prestin, is a membrane transporter
Homology model: Gorbunov et al., 2014
Genomic identity: SLC26A5 - a low efficiency Cl-HC03 antiporter? (SLC26 is a superfamily of anion-bicarbonate exchangers)
Originally reported by Zheng et al, 2000 16
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Prestin transfected HEK cells
Zheng et al,. 2000
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Prestin KO mouse does not have amplification
WT prestin KO
• = post-mortem
Liberman et al., Nature 2002 Mellado Lagarde et al., Curr Biol 2008
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2015: crystal structure: of a bacterial SLC26 (from a thermophilic bacterium, Deinococcus geothermalis)
Geertsma, et al., Nat. Struct. Mol. Biol. 2015 From Reithmeier & Moreas with permission
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Mammalian prestin (SLC26A5) is a tetramer = dimer of dimers
outside
nanometres membrane
cytoplasm
Mio et al, J Cell Biol. 2008
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The SLC26 superfamily and relatives in Drosophila & C. elegans
They are bicarbonate exchangers
(Red blood cells have bicarbonate exchangers in the SLC2 family) Mount and Romero Eur J Physiol (2004) 21
A simple model for prestin action
extended
=0exp(-bV) fast 1 μs
compact
- - = intracellular anion (Cl or HCO3 )
‘Molecular crowding’ leads to macroscopic effects.
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Out In
slow
expanded state
=0exp (-bV) fast
compact state
conventional 4 state carrier model
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A nagging problem
Ikeda et al, 1991: OHCs regulate their intracellular pH Prestin is a member of the SLC26A anion-bicarbonate family of transporters
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- - Cl controls HCO3 loading in prestin expression systems
prestin pHluorin
membrane peptide - pHluorin
pH pH pH ?
- - - CO2 HCO3 HCO3 Cl
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HCO3 is transported by prestin under Cl control Cl = 6mM Cl =140 mM
Mistrik et al, 2012
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Out In
slow
expanded state
=0exp (-bV) fast
compact state
= aspirin
conventional 4 state carrier model -1 low turnover rate ~900 s (=0.1% of other Cl-HCO3 transporters)
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Conclusions
1) The mammalian OHC is an ultrafast actuator
2) The ‘motor’ molecule is also low efficiency Cl -HCO3 exchanger
3) The ‘motor’ arises from part of a transport cycle and creates a ‘cochlear amplifier’
Unresolved: how is it inserted and (?) is there turnover ? what is the mechano-enzyme structure of prestin? how is it coupled into the cell cytoskeleton?
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